The present invention relates to the control of the air-fuel ratio of an engine and, more particularly, to the shift of the air-fuel ratio (.lambda.).
A feedback control method is, for example, disclosed in the specification of Japanese Patent Laid-Open No. 48738/1977, which includes the steps of detecting the condition of exhaust gas from an engine by an exhaust gas sensor, integrating the output of the sensor while changing the integration direction in accordance with the detected exhaust gas condition, and correcting the amount of fuel supplied to the engine on the basis of the result of the integration.
According to the method disclosed in the above specification, a predetermined value is added to or subtracted from the result of the integration simultaneously with the change of integration directions. The response of control is improved by the addition or subtraction thus carried out. This prior art method, however, has the disadvantage that it is extremely difficult to adjust the air-fuel ratio to match the engine speed.
As the engine speed increases, the lean-rich inverting time of the exhaust gas condition is decreased. In consequence, the rate of the degree of influence by the delay in control changes, so that, as the engine speed changes, the air-fuel ratio is offset in one direction.
Feedback control needs to be carried out in consideration of the above phenomenon, and it is difficult to adjust the air-fuel ratio so to match the engine speed.
In order to solve this problem, it has been proposed to vary the integral slope or integration gradient of the feedback control value; however, this produces additional problems in that variation of the integration gradient affects the ability to adequately reduce harmful components in the exhaust gas.